I'm kind of a beginner myself, but at least with adjectives you usually don't use it in positive sentences at all. With 不, you do use it with long adjectives but not with short ones. 我的字不好. vs. 他的西瓜不是新鮮. If you do use it, it emphasizes what you're saying, like "(but) you ARE beautiful!"

This helped me https://www.fluentu.com/blog/chinese/2015/07/09/shi-chinese/

Is nobody monitoring these questions? 3 weeks and no reply! I have the same question.

The main function of the verb 是 is to serve as a copula of two nominals, i.e. tell us that "sth is sth". It cannot be used with adjectives as a copula, as adjectives in Chinese work like verbs, e.g. 大 "big" is literally "to be big" with that English "to be" already being expressed within it.

Yes. The 的 of possession can basically only be dropped when talking about family members; else should be used. It is better to always use it to not mistake.